The present invention relates generally to a connector and, more specifically, to an optical coupler for single optical fibers.
The employment of fiber optic cables or light guides, also sometimes referred to as optical communication fibers, for the transmission of information-bearing light signals, is now an established art. Much development work has been devoted to the provision of practical low-loss glass materials and production techniques for producing glass fiber cables with protective outer claddings or jackets. The jackets make them resemble ordinary metallic-core electrical cable upon superficial external inspection. Obviously, if fiber optic cables are to be used in practical signal transmission and processing systems, practical connectors for the connection and disconnection of fiber optic cables must be provided.
Some references will now be given for background in the state of fiber optic art in general. An article entitled, "Fiber Optics," by Narinder S. Kapany, published in Scientific American, Vol. 203, pgs. 72-81, November 1960, provides a useful background in respect to some theoretical and practical aspects of fiber optic transmission.
Of considerable relevance to the problem of developing practical fiber optic connectors is the question of transfer efficiency at the connector. Various factors, including separation at the point of abutment, and lateral separation or axial misalignment, are among the factors effecting the light transfer efficiency at a connector. In this connection, attention is directed to the Bell System Technical Journal, Vol. 50, No. 10, December 1971, specifically to an article by D. L. Bisbee, entitled, "Measurement of Loss Due to Offset, and End Separations of Optical Fibers." Another Bell System Technical Journal article of interest appeared in Vol. 52, No. 8, October 1973, and was entitled, "Effect of Misalignments on Coupling Efficiency on Single-Mode Optical Fiber But Joints," by J. S. Cook, W. L. Mammel, and R. J. Grow.
Fiber optic bundles are normally utilized for only short transmission distances in fiber optic communications networks. On the other hand, fibers are used individually as optical data channels to allow transmission over many kilometers. At present, most fiber optic cables are multi-fiber bundles due to the less stringent splicing requirements, greater inherent redundancy, and higher signal-to-noise ratio. The difficulty in achieving connections between single fibers which are insensitive to axial misalignment problems has created an obstacle to the use of long run single data transmission systems.
Therefore, a connector or coupler is required to eliminate lateral tolerances if low-loss connections are to be obtained in the use of single fiber optical transmission arrangements, "V" groove and metal sleeve arrangements have been used to interconnect single fibers. Reference is made to U.S. Pat. No. 3,768,146 which discloses a metal sleeve interconnection for single fibers.
Another known device, as shown in U.S. Pat. No. 3,734,594, utilizes a deformable annular core having pressure plates at the ends. The fiber ends are inserted into the core and an axial force is applied to the plates to deform the core radially, thereby aligning and securing the fibers.
These prior devices, however, do not readily provide sufficient accuracy for joining and aligning small diameter cores of optical fibers.
Copending application of Charles K. Kao entitled, "Precision Optical Fiber Connector," Ser. No. 613,390, filed Sept. 15, 1975 now U.S. Pat. No. 4,047,796, assigned to the assignee of the present application, discloses a single optical fiber connector in which the ends of mating fibers are precisely aligned and coupled together in the interstice between three like contacting cylindrical rods. The rods are mounted along and around the fibers within an adjustable connector assembly. Means is provided for expanding the interstice to insert the fiber ends and for clamping the rods in position around the fibers. Copending application of Charles K. Kao entitled, "Precision Surface Optical Fiber," Ser. No. 629,210 filed Nov. 5, 1975 now U.S. Pat. No. 4,057,322, assigned to the assignee of the present application, discloses an optical fiber in which the plastic cladding thereof is formed with three rounded indentations along its surface and a thin metal ferrule is formed around the cladding at the mating end of the fiber. A pair of such fibers may be aligned in a three rod arrangement of the type mentioned above.
A hermaphroditic connector for coupling a pair of single optical fibers is disclosed in copending application of Ronald L. McCartney entitled, "Single Optical Fiber Connector," Ser. No. 629,004, filed Nov. 5, 1975 now abandoned, also assigned to the assignee of the present application. The connector comprises a pair of connector members each containing at least one single optical fiber terminated by a termination pin. The pin includes a metal eyelet crimped about the optical fiber in three places providing three, spaced, curved indentations which centrally position the fiber in the pin. When the connector members are mated, the mating termination pins are positioned so that the indentations therein are generally aligned. Three arcuate cam or spring members are forced into the indentations in the mating termination pins to accurately laterally align the pins and, hence, the optical fibers therein.
In copending application of Ronald L. McCartney entitled, "Single Optical Fiber Connector," Ser. No. 680,171, filed Apr. 26, 1976, there is disclosed a single optical fiber connector comprising a base plate having a V-groove in its upper surface having a transverse cross section of an equilateral triangle. Two sets of three equal diameter cylindrical rods lie in the groove, each defining an interstitial space therebetween which receives an optical fiber. The sets of rods have mating end faces which abut each other in the groove. A compression plate is mounted over the base plate to arrange the rods in the V-groove so that the centers of the rods are disposed at the vertices of the same equilateral triangle whereby the fibers in the interstitial spaces between the rods become precisely laterally aligned. Such connector arrangement is particularly suited for a flat cable having single optical fibers.
In copending application of R. L. McCartney et al, entitled, "Single Optical Fiber Connector Utilizing Elastomeric Alignment Device," Ser. No. 680,170, filed Apr. 26, 1976 now U.S. Pat. No. 4,056,305, there is disclosed a single optical fiber connector which incorporates a deformable elastomeric alignment element having a bore therethrough. A pair of contacts are mounted lengthwise in the bore. The contacts embody like sets of three equal diameter cylindrical rods. Preferably, the rods are formed of plastic and are integral with a plastic body of the contact. The adjacent cylindrical surfaces of the rods of each contact provide a tricuspid interstitial space for receiving an optical fiber. The sets of rods of the contacts have mating end faces which abut each other when the contacts are pushed into the opposite ends of the bore in the alignment element. The relative dimensions of the two sets of rods and the bore in the elastomeric alignment element are selected so that the region of the element surrounding the mating end faces of the rods is strained to exert a radially inwardly directed compressive force urging the rods of each set inwardly. Such inward compression of the rods causes the adjacent cylindrical surfaces thereof to engage each other and the fiber disposed therebetween so that the centers of the three rods of each contact are disposed at the vertices of an equilateral triangle, whereby the fibers in the contacts become precisely laterally aligned. Such coupling arrangement is suited for axially mated connectors.
The three-rod contact alignment approach discussed hereinabove has been found to suffer some problems. Normally, the optical fiber mounted in the interstitial space defined by the three rods of the contact is recessed slightly behind the mating end faces of the rods so that when two mating contacts are abutted under axial compression force, the fibers therein will not engage each other but will be slightly spaced apart. Since the alignment rods are formed of plastic, the rods experience axial creepage due to the axial compression force applied to the contacts to maintain them in mating engagement. The creepage of the rods causes the fibers mounted therebetween to be exposed at their ends with the result that the fibers in the mating contacts eventually touch each other. The engagement of the fibers causes the fibers to chip at their end faces, resulting in light transmission losses. Also, frequently the fibers will buckle and crack under the axial compressive loads. The foregoing problem of axial creepage of plastic rods may be overcome by the use of hard metal rods on the contact, but this requires additional parts and, therefore, increases manufacturing costs.
It is therefore, the object of the present invention to overcome the aforementioned disadvantages of the three-rod alignment approach but still provide a controlled, accurate alignment of mating single optical fibers in a manner which will minimize light transmission losses, which is easily manufactured and assembled, relatively inexpensive, and practical for commercial field use.